In general, when electrical apparatus such as a motor and a transformer is to be produced using an electrical steel sheet, firstly a coiled electrical steel sheet is blanked into a predetermined shape, and the blanked electrical steel sheets are laminated and then fixed to form an iron core. Next, a copper wire is wound around the teeth and others of the iron core, and then the iron core is impregnated with a varnish and the iron core is sprayed with a powder coating composition for example, which is baked and dried. Further, a terminal for copper wire connection, a flange, bearings and others are attached to the dried iron core, and then the iron core is secured to a casing. In this manner, motors, transformers, and others are produced.
Electrical steel sheets are provided with an insulation coating on their surfaces to prevent current flow between the laminated steel sheets so as to reduce iron loss and improve magnetic properties.
Insulation coatings applied to the surface of an electrical steel sheet need to exhibit good insulating properties, and in addition, need to exhibit good coating properties in addition to good insulating properties, such as weldability, smoothness, adhesion, thermal resistance, and coating compatibility, in order to improve working efficiency for working of the electrical steel sheet.
Known examples of insulation coatings of electrical steel sheets include coatings based on an organic resin, coatings based on a salt of an inorganic acid such as a chromate salt or a phosphate salt, and coatings based on a mixture of a salt of an inorganic acid and an organic resin.
In general, insulation coatings based on an organic resin have low thermal resistance and insulation coatings based on a salt of an inorganic acid tend to delaminate from the steel sheet during working, and for this reason, conventionally, coatings based on a mixture of a salt of an inorganic acid and an organic resin are frequently used as insulation coatings.
Examples of technologies related to the insulation coating of an electrical steel sheet are as follows. JP50-15013, an old publication, discloses a technique for forming an insulation coating using a treatment solution based on a bichromate salt and an emulsion of an organic resin such as a vinyl acetate-acrylic resin copolymer, a butadiene-styrene copolymer, or an acrylic resin.
JP 2002-317276 discloses a technique related to an insulation coating composition having improved wettability. The insulation coating composition is formed by mixing a predetermined amount of non-ionic or anionic surfactant having a predetermined HLB (Hydrophile-Lipophile Balance) value with an insulation coating solution containing a phosphate salt and a chromate salt.
In recent years, with increasing awareness of environmental issues, technologies have been developed for forming an insulation coating without using an aqueous solution of hexavalent chromium-containing chromic acid. Such technologies are disclosed, for example in JP06-330338. It discloses a technique in which at least one among a phosphate salt having a specific composition, a boric acid, and colloidal silica is mixed with an emulsion of an organic resin having a specific particle size, at a specific mixing ratio, and the mixture is baked on a steel sheet. This technique uses a treatment solution free of a chromium compound but can achieve good coating properties comparable to those of conventional chromium compound-containing insulation coatings, and in addition, can retain excellent smoothness after stress relief annealing.
Furthermore, developments have also been made for such chromic acid-free insulation coatings for their influence on the workability of the electrical steel sheets. For example, JP11-80971 discloses a technique for improving blankability of an electrical steel sheet by applying and baking, on an electrical steel sheet, a treatment solution based on a metal phosphate and an organic resin and in which the carbon i s peak and the phosphorus 2 s peak have specific intensities as measured by photoemission spectroscopy. JP11-80971 also discloses that blankability of an electrical steel sheet can be further improved by adding a specific amount of aqueous organic compound having a boiling point or sublimation temperature of 100° C. or more to a treatment solution based on a metal phosphate and an organic resin.
JP2002-47576 discloses techniques related to a treatment solution for insulation coatings of electrical steel sheets which contains, at a specific ratio, a monobasic phosphoric acid salt of specific metal ions and a phosphonic acid compound, and a method for treating an electrical steel sheet using the treatment solution for insulation coatings.
JP2008-31245 discloses a technique related to a lubricant composition capable of imparting high corrosion resistance to various types of steel sheets, capable of being readily removed with an alkaline degreasing agent, and capable of being used as a metal working oil.
JP2010-261063 discloses a technique related to a treatment solution for forming insulation coatings of electrical steel sheets. The treatment solution contains a monobasic phosphoric acid salt of a multivalent metal, a chelating agent, a polyamine, and a synthetic resin.